result in rust

result.rs

fn foo(x: int) -> Result<int, ~str> {
    if x == 5 {
        Ok(5)
    } else {
        Err(~"x was not five")
    }

}

fn main() {
   match foo(5) {
       Ok(num) => println!("Success! {:d}", num),
       Err(msg) => println!("failure: {:s}", msg)
   }
   match foo(6) {
       Ok(num) => println!("Success! {:d}", num),
       Err(msg) => println!("failure: {:s}", msg)
   }
}